It has long been known to use devices to help simplify the production of suitable molds for the production of dental restoration parts. For example, it is known according to DE 539 734 to use what is known as a melting crucible former for forming the lining material, which has a high content of gypsum and to that extent corresponds to a muffle, a substantially conical base plate and a lining ring being used there, the latter as a sleeve.
This solution already allows a mold cavity, which is initially modelled in wax as a positive mold, to be created in the muffle, and then serve after curing as a pressing space for the dental restoration.
A comparatively much improved device for making a muffle is known from DE 900 17 40 U1. In the case of this solution, a central mold attachment is provided, which projects from an end wall part and allows models to be grown on its tooth supporting surface. After hardening of the muffle, to provide the dental restoration it is necessary for the mold attachment together with the end wall part to be pulled off, so that a certain strength is necessary there to ensure the desired clean separation of muffle and wax.
Specifically in the case of a cylindrical construction of the mold attachment, this solution requires a certain strength of the mold attachment, and consequently also a comparatively great wall thickness of the mold attachment, which is for example much greater than the wall thickness of the sleeve which surrounds the muffle.
To facilitate the pulling out of such a mold attachment, the mold attachment may be turned, and at the same time pulled out. This also facilitates the detachment without impairing or damaging the muffle.
However, such turning is only possible in the case of circular-symmetrical forms of the mold attachment, and also only when only one mold attachment is provided. In this connection, it has also been proposed to create special pulling devices for the mold attachments that are connected to pulling tools. Although a mold attachment can be removed from the muffle with such a pulling device, the comparatively high forces that must overcome the adhesion between the mold attachment and the muffle put at risk the intactness of the muffle, so that this solution has not been adopted.
Furthermore, it has also been proposed to design such mold attachments not in a cylindrical manner but in a slightly conical manner, or possibly even slightly spherical, to facilitate demolding. Although this can assist demolding, this solution has also not been adopted, since it is not possible in this way to use the feed channel created in the muffle as a press channel, because otherwise, in view of the conical form, the cylindrical press blank deviates to the side, which leads to serious blockages and in any event means that it is not ensured that dental material in a semifinished state will pass by the sides of the press ram, so that the mold cavity is not completely filled.
Furthermore, various other attempts have been undertaken to improve the making of muffles. For instance, it has been attempted to increase the diameter, or at least the cross-sectional area, of a press channel, in order also to create larger dental restoration parts, or possibly to create a number of dental restoration parts. However, an increase in the cross-sectional area also requires a correspondingly increased pressing pressure, so that then the wall thickness of the muffle also has to be increased to withstand the increased pressing pressure. Moreover, it has been found that an increased pressing pressure more easily leads to “compression” of the muffle, and consequently to reduction of the dimensional accuracy, so that, all in all, increasing the cross-sectional area of the press channel has not proven to be a promising prospect.